Induction furnace



Sept. 22, 1931. N, R, DAVlS 1,823,970

INDUCTION FURNACE Filed Sept. 25. 1929 Nev/'lle R. Dac/IS TToRNr-:Y

Patented Sept. 22, 1931 UNITED STATES PATENT OFFICE NEVILLE RYLANDDAVIS, F WOODCOURT,` BROOKLANDS, ENGLAND, ASSIGNOR T0 WESTINGHOUSEELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIAApplication led September 26, 1929, Serial No.

My invention relates to electric furnaces and particularly to electricinduction furnaces.

An object of my invention is to provide a relatively simple and highlyeiiicient protecting means for 'an electric induction furnace.

Another object of my invention is to provide a relatively simple meansfor preventing electrical shock to an operator in case of molten -metalleaking from the crucible of an induction furnace.

In practicinnf my invention, I provide a furnace assembly including acrucible, an

` induction coil located therearound and other suitable elements, suchas a casing, and I provide a laminated metallic structure orelectric-conducting screen connected to ground and located eitherbetween the crucible and the winding or between the winding and thecasing.

In the single sheet of drawings,

Figure 1 is a view, in vertical section, of an electric inductionfurnace embodying my invention.

Fig. 2 is a fragmentary view, in section, on a somewhat enlarged scale,of an induction coil with which is associated the particular deviceembodying my invention, and

Fig. 3 is a view, in perspective, of a modication of the deviceparticularly embodying my invention.

Referring more particularly to Fig. 1 of the drawings, I have thereillustrated an induction furnace of the surrounded-pool type, asembodying an outer metal casing 12 which may have an open top and whichmay have any other mechanical elements associated therewith to permit offilling and emptying a refractory crucible 13 located within the casing.

An induction coil 14 is shown schematically only as embodying aplurality of turns of a suitably insulated electric vconductorsurrounding the crucible 13 and coaxial therewith. A quantity ofelectric and heat-insulating granular material 16, such as zirconiumsand, may be used within the casing 12 to support not only the inductioncoil but also the crucible. The construction and assembly shown in Fig.1 of the drawings are for illus- INDUCTION FURN ACE 395,229, and inGreat Britain october 9, 1928.

trative purposes only and it is to be understood that such modificationsmay be made in these parts as may be desirable, as my particularinvention does not embody these elements.

A cover 17 of `substantially annular shape is provided to cooperate withthe upper rim of the casing 12 and with the crucible 13 to close thecasing.

While no terminal members or terminal leads have been shown for theinduction coil 14, it is to be understood that any structure suitablefor such purposes may be employed.

In the Operation of relatively large induction furnaces of this type, itis sometimes desirable, and even necessary, that relatively high-voltageenergization of the induction coil be used, and as it is possible, inthe operation of the furnace, that the charge within the crucible orelectrically-conductingl parts of the furnace casing may accidentallybecome connected to the induction coil and, consequently, may besubjected to a relatively high alternating electric potential which mayconstitute a danger to workmen who are operating the furnace.

The element particularly embodying my invention includes a laminated orforaminous screen of electrically-conducting material of relatively highinherent electrical resistance which is located either closely adjacentto the crucible and between the crucible and the coil or between thecoil and an outer casing.

In the construction shown in Fig. 1 of the drawings, the ground shieldcomprises a plurality of relatively thin and narrow strips 18 of asuitable metal having a high electrical resistance, The'material of thestrips may be some form of nickel-steel or nickelchromium-steel alloywhich may be purchased in the open market. The strips 18 are so locatedthat their main or body portions extend longitudinally or axially of thecrucible and may be located a relatively small distance apart, the lowerends thereof being made narrower and bent under the crucible,substantially as shown in Fig. 1 of the drawings. The strips also helpto support the crucible and strengthen the same.

CII

A plurality of peripherally extending spaced and alined plates 19, whichmay be of the same material, and are mechanically secured against theupper ends of certain of the strips 18 either by rivets or screws or bywelding. It is essential that the construction of the ground shield orbasket be such that there is no closed electrical circuit surroundingthe charge in order that the energy expended in the generation of eddycurrents therein may-be relatively small and the temperature risetherefor also be relatively small. A ground wire 2'1 is shownschematically only in Fig. 1 of the drawings.

If the construction shown in Fig. 1 is employed, it is desirable thatthe width of the strips, expressed in centimeters, shall be below acertain value, and a careful analysis of this problem has indicated thatthe desirable relationship expressed in the formula at which the coil isenergized.

Fig. 2 shows an induction coil, partly in section, having a plurality ofturns of a composite conductor including a solid intermediate section 23and a tubular outer section 24 in heat interchanging relation with eachother. Each turn has located adjacent to the inner edge portion thereofa strip or bar 26 of substantially the same type as bar 18 in Fig. 1,but coextensive with the inner periphery of the portion 23 and separatedtherefrom by narrow strips of electric-insulating material 27, each turnbeing enclosed by suitable insulating material 28. y

Fig. 3 shows a still further modification of the device embodying myinvention. A plurality of strips 31 of relatively thin material have oneend of each strip connected to an electric-conducting strip 32 extendingat right angles to the strips 31, the other ends of the respectivestrips 31 being left unconnected, so that no closed electric circuitshall be provided thereby. In this case, the strips 31 extendperipherally of the crucible instead of axially thereof, but, in otherrespects, the action is identical. A suitable ground connection 33 isprovided which may be of the usual kind.

It is obvious that a foraminous metal screen may be employed in place ofthe strips of electric-conducting material to produce substantially thesame results.

It may happen that a crucible will crack slightly or that minutefissures may appear in the walls and, when certain metals are to bemelted-in the crucible, a very minute fissure will permit this metal toleak or seepY through the crucible, and the molten metal yma in time,reach the induction coil, in

whlch case, a dangerously hi h potential will be impressed upon thecruel le and its contents which may result in injury to workmen. The useof my grounded screen will overcome' this danger and make the device andfurnace structure safe to operate under all conditions.r

Various modifications may be made in the device embodying my inventionwithout departing from the spirit and scope thereof, and I desire,therefore, that only such limitations shall be placed thereon as areimposed by the prior art or are set forth in the appended claims.

I clalm as my invention:

1. In an electric induction furnace having a crucible, an induction coilsurrounding thev crucible, and a casing therearound, -an earthedelectric-conducting screen located within the casing and between it andthe crucible.

2. In an electric induction furnace havin a crucible and an inductioncoil surroun ing the crucible, an earthed electric-conducting screenlocated between the crucible and the coil.

3. In an electric induction furnace including a crucible and aninduction coil therearound, an earthed laminated metal shieldsurrounding the crucible.

' 4. In an electric induction furnace including a crucible and aninduction coil therearound, an earthed discontinuous metal shieldsurrounding the crucible and ineluding a plurality of relatively narrowstrips.

5. In an electric induction furnace including a crucible and aninduction coil therearound, an earthed discontinuous metal shieldsurrounding the crucible and including a plurality of relatively narrowstrips, the individual strips being such that the ratio expressed by theformula is less than 2, where W is the width in centimeters of thestrips. R is the resistivity of the material in the strips expressed inelectromagnetic units and F equals 2r times the frequencv.

6. In an electric induction furnace having a crucible, an induction coilsurrounding the crucible, and a casing' therearound, means immediatelysurrounding the crucible for preventing leakage of material from causingthe potentials of the induction coil to be impressed on said casing.

In testimony whereof, I have hereunto subscribed my name this 11th dayof September, 1929. NEVILLE RYLAND DAVIS.

